Today's integrated circuits consist of as many as a billion transistors, a large number of input/output pins, and provide extensive functionality. To support the design, simulation, verification, place-and-route, and layout of these integrated circuits at the system, chip and logic levels, the integrated circuit (IC) industry has developed highly robust, well-established and standardized computer-aided design (CAD) tools, particularly electronic CAD (E-CAD) and methodologies. The E-CAD tools support digital, analog or mixed signal integrated electronic circuits. Generally, IC designers use libraries of circuit, gate and/or logic elements that are available through the well-known E-CAD software tools, or develop “custom” tools in-house to meet specific needs. The power of the standardized E-CAD tools has significantly fueled the growth and maturity of the IC industry.
Integrated circuit designs may employ custom, semi-custom, or a combination of custom and semi-custom design methodologies. “Custom” refers to the creation of a new physical layout for each design. Semi-custom refers to the use of predefined circuit elements, such as “gate array” and “standard cell” elements. Gate arrays employ a set of pre-defined functions fabricated on a semiconductor wafer that may be later interconnected to implement a design. Standard cell technologies provide a library of low-level circuit functions each having a predefined physical layout. The predefined physical layout (or “cells”) typically have a common dimension such as width or height such that they may be placed in rows and blocks, the order determined by functions to be implemented and routing of interconnect between cells or groups of cells.
In developing an integrated circuit, a designer may partition a design into various functional blocks and then design circuitry for each functional block or re-use a design for a functional block if a previous design meets size, power and performance criteria. Circuit design most frequently employs a hardware descriptive language (HDL) that specifies circuit elements and the connection between elements. Verilog® is a commonly used HDL and is the topic of IEEE Std 1364. Verilog is a registered trademark of Cadence Design Systems, headquartered in San Jose, Calif. Verilog may be used to specify the initial design, to provide input to simulation and synthesis tools, and to check post-layout operation. A version of HDL suitable for use with analog circuits (A-HDL), or for Very high speed integrated circuit HDL (VHDL)—including VHDL-AMS for analog/mixed signal applications, are also known in the art.
At times, the pre-defined set of cells of a standard cell library may not provide a desired function, or may not provide the speed, size or power consumption desired. In these circumstances, new cells may be created, or a custom block of logic incorporating the desired function and capabilities may be designed. The design of the custom block of logic may employ “SPICE” (Special Programs for Interactive Circuit Elements) to specify and simulate the design. Some product versions of SPICE support both logical and timing simulation. However, SPICE simulation is extremely slow when compared to simulation employing an HDL netlist model. When designs include both standard cell and custom logic sections, a problem arises when attempting to simulate the entire design. The custom logic may exist simply as a “black box” wherein operation of standard cell and custom logic are separately simulated; simulation comprising both sections is not performed. A behavioral model, such as may be written in the C programming language, may be employed for function simulation, but such models do not allow for timing analysis.
Besides the problem of standard cells vs. custom logic, more and more integrated circuits are being formed that include both electronic circuit elements and optical circuit elements, particularly in light of the use of relatively thin silicon layers on an SOI substrate to support both types of elements in a monolithic structure.
The optics industry is in a similar state today as the electronic IC industry was in the 1960's. As such, today's optics industry lacks a common technology platform to integrate different components (building blocks) to make a subsystem. As a result, the current optical industry at large has a highly “un-integrated” approach for designing, simulating and verifying the mostly discrete optical components and optical systems. The few existing design, simulation and verification tools for optical elements tend to be overly specific to a particular type of optical device, or a system of optical components. Indeed, these tools have generally been developed for III-V based optical devices, not the silicon components used in the inventive integrated arrangement.
Recently, however, many factors have come together to make the integration of optical and electrical circuits a reality, allowing for optics and electronics to be incorporated on a monolithic platform using standard CMOS processing technology (as widely accepted by the IC industry). This approach towards silicon-based IC and optics integration aspires to leverage the discipline, maturity and capability of the IC industry into the monolithic platform. Recent efforts to demonstrate the feasibility of this approach are highly promising. To support this effort of integration, however, there is a need to design, simulate and verify both the optical and electronic components, preferably using the same tools during the design and development phases.
E-CAD tools, used for the design and development of traditional electronic integrated circuits, utilize various types of parameters that essentially characterize and model the electronic integrated circuits. These parameters can be the signal inputs, outputs, clock signal, time delays, load, voltages, and so on. The characterization of the electronic circuit elements enables the designers to design, simulate and verify the circuits prior to mask and fabrication. These parameters can be in analog or digital format, and are readily available within various E-CAD software libraries.
Optical modeling, as mentioned above, has heretofore been limited to use with traditional III-V-based optical devices. With the advent of silicon-based optical devices, and the integration of optical (i.e., passive optical devices), electronic and opto-electronic (i.e., active optical devices) components, the need has arisen for a methodology to simplify the fabrication steps associated with such a monolithic design.